Plate-like composite material containing polytetrafluoroethylene and filler

ABSTRACT

Provided is a composite material that shows a low specific dielectric constant, and that hardly causes an appearance failure or changes in characteristics when exposed to, for example, a treatment liquid to be used in the production of an electronic circuit board. Specifically, a plate-like composite material including polytetrafluoroethylene and a predetermined filler, and satisfying a predetermined condition serves as a composite material that shows a low specific dielectric constant, and that hardly causes an appearance failure or changes in characteristics even when exposed to, for example, a treatment liquid to be used in the production of an electronic circuit board.

This is a divisional application of U.S. application Ser. No. 16/615,623filed on Nov. 21, 2019, which is a National Stage Application filedunder 35 U.S.C. § 371 of International Application PCT/JP2018/020672filed on May 30, 2018, which is based upon and claims the benefit ofpriority from the prior application of U.S. Provisional Application No.62/560,432 filed Sep. 19, 2017, which is based upon and claims thebenefit of priority from the prior application of U.S. ProvisionalApplication No. 62/567,966 filed Oct. 4, 2017, which is based upon andclaims the benefit of priority from the prior application of U.S.Provisional Application No. 62/619,562 filed Jan. 19, 2018 which ishereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a plate-like composite materialsuitable for an electronic circuit board or the like.

BACKGROUND ART

Due to advance of electronic technology, electronic devices, such ascomputers and mobile communication devices, each using the highfrequency band are increasing. For a wiring board and a multilayerwiring board for high frequencies to be used for such electronicdevices, a low specific dielectric constant is generally required, andnonpolar resin materials, such as polyethylene, polypropylene,polystyrene, and polytetrafluoroethylene, are utilized.

For example, a composite material obtained by blending a fluoropolymermatrix with hydrophobic coated hollow inorganic microspheres has beenproposed as a wiring board material excellent in mechanical, thermal,and electrical properties (see PTL 1). In addition, a composite materialobtained by blending boron nitride or the like into a fluoropolymer hasbeen proposed as a printed wiring board material that has a small fillercontent and is easily subjected to drilling (see PTL 2).

RELATED ART DOCUMENT Patent Document

PTL 1: JP-A-HEI06(1994)-119810

PTL 2: JP-A-HEI03(1991)-212987

SUMMARY

Various chemicals have been used in a production process for anelectronic circuit board. However, for example, when the board or amaterial therefor is exposed to a treatment liquid having highpermeability, the treatment liquid may permeate the inside of the boardor the material to cause an appearance failure of the board or changesin characteristics thereof. In particular, further attention needs to bepaid to a board that is blended with a large amount of a filler forimproving its dimensional stability or strength, or that is increased inporosity for reducing its dielectric constant because the treatmentliquid is liable to permeate the board.

That is, the present disclosure provides a composite material that showsa low specific dielectric constant, and that hardly causes an appearancefailure or changes in characteristics when exposed to, for example, atreatment liquid to be used in the production of an electronic circuitboard.

In order to achieve the object, the gist of the present disclosureincludes the following items [1] to [5].

[1] A plate-like composite material, including:

-   -   polytetrafluoroethylene; and    -   a filler,    -   wherein the filler contains porous inorganic fine particle        aggregates each formed by aggregation of inorganic fine        particles having an average primary particle diameter of from 5        nm to 200 nm,    -   wherein the composite material has a porosity of 35% or more,        and    -   wherein the composite material has a critical liquid-repellent        tension determined by the following wetting tension test of 34.0        mN/m or less:

[Wetting Tension Test]

-   -   a test object is immersed in each of test mixtures, which        correspond to test mixtures described in JIS K 6768:1999 of        Japan Industrial Standards, and which have wetting tensions at        23° C. of 22.6 mN/m, 25.4 mN/m, 27.3 mN/m, 30.0 mN/m, 31.0 mN/m,        32.0 mN/m, 33.0 mN/m, 34.0 mN/m, 35.0 mN/m, 36.0 mN/m, 37.0        mN/m, 38.0 mN/m, 39.0 mN/m, 40.0 mN/m, 41.0 mN/m, 42.0 mN/m,        43.0 mN/m, 44.0 mN/m, 45.0 mN/m, 46.0 mN/m, 48.0 mN/m, and 50.0        mN/m, at 25° C. for 1 minute to confirm whether or not each of        the test mixtures permeates the test object, and a numerical        value of a wetting tension of a test mixture having the smallest        wetting tension out of the test mixtures that have not permeated        the test object is determined to be a critical liquid-repellent        tension of the test object.        [2] The composite material according to the above-mentioned item        [1], wherein the filler has a BET specific surface area of from        30 m²/g to 240 m²/g.        [3] The composite material according to the above-mentioned item        [1] or [2], wherein a content of the filler is 40 parts by mass        or more with respect to 100 parts by mass of a total of the        polytetrafluoroethylene and the filler.        [4] The composite material according to any one of the        above-mentioned items [1] to [3], wherein the filler has an        apparent specific gravity of 100 g/L or less.        [5] The composite material according to any one of the        above-mentioned items [1] to [4], wherein the composite material        is used for an electronic circuit board.

According to the present disclosure, the composite material that shows alow specific dielectric constant, and that hardly causes an appearancefailure or changes in characteristics when exposed to, for example, atreatment liquid to be used in the production of an electronic circuitboard can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an image of a composite material includingpolytetrafluoroethylene and hollow inorganic fine particles (filler)taken with a scanning electron microscope (SEM) (photographs serving asalternatives to drawings).

FIGS. 2A and 2B are images of a composite material according to oneaspect of the present disclosure taken with a scanning electronmicroscope (SEM) (images in each of which a three-dimensional finenetwork structure is formed by polytetrafluoroethylene and porousinorganic fine particle aggregates each formed by the aggregation ofinorganic fine particles having an average primary particle diameter offrom 5 nm to 200 nm, in which FIG. 2A is a sectional image in athickness direction (at a magnification of 50,000), and FIG. 2B is asectional image in a surface direction (at a magnification of 50,000))(photographs serving as alternatives to drawings).

FIG. 3 is an image of a porous inorganic fine particle aggregate, whichis formed by the aggregation of inorganic fine particles having anaverage primary particle diameter of from 5 nm to 200 nm, taken with ascanning electron microscope (SEM) (a photograph serving as analternative to a drawing).

DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure are hereinafter described indetail. The present disclosure is not limited to these embodiments.

Composite Material

A composite material according to one aspect of the present disclosure(hereinafter sometimes abbreviated as “composite material”) is aplate-like composite material including polytetrafluoroethylene and afiller, wherein the filler contains porous inorganic fine particleaggregates each formed by the aggregation of inorganic fine particleshaving an average primary particle diameter of from 5 nm to 200 nm(hereinafter sometimes abbreviated as “inorganic fine particleaggregates”), wherein the composite material has a porosity of 35% ormore, and wherein the composite material has a critical liquid-repellenttension determined by the following wetting tension test (hereinaftersometimes abbreviated as “critical liquid-repellent tension”) of 34.0mN/m or less.

[Wetting Tension Test]

A test object is immersed in each of test mixtures, which correspond totest mixtures described in JIS K 6768:1999 of Japan IndustrialStandards, and which have wetting tensions at 23° C. of 22.6 mN/m, 25.4mN/m, 27.3 mN/m, 30.0 mN/m, 31.0 mN/m, 32.0 mN/m, 33.0 mN/m, 34.0 mN/m,35.0 mN/m, 36.0 mN/m, 37.0 mN/m, 38.0 mN/m, 39.0 mN/m, 40.0 mN/m, 41.0mN/m, 42.0 mN/m, 43.0 mN/m, 44.0 mN/m, 45.0 mN/m, 46.0 mN/m, 48.0 mN/m,and 50.0 mN/m, at 25° C. for 1 minute to confirm whether or not each ofthe test mixtures permeates the test object, and a numerical value of awetting tension of a test mixture having the smallest wetting tensionout of the test mixtures that have not permeated the test object isdetermined to be a critical liquid-repellent tension of the test object.

The inventors have made investigations on plate-like materials suitablefor an electronic circuit board and the like, in particular, dispersionstrengthened composite materials each obtained by mixing a polymermaterial serving as a matrix with particles. In the course of theinvestigations, the inventors have confirmed that, when hollow inorganicfine particles are used as a filler like the composite materialdescribed in PTL 1, the hollow inorganic fine particles are broken in aproduction process for a composite material (see images taken with ascanning electron microscope (SEM) shown in FIG. 1), and hence theirfunctions cannot be sufficiently exhibited in some cases. In addition,the inventors have revealed that, in the case where the inorganic fineparticle aggregates are used as a filler, the aggregates are not brokeneven when subjected to treatment such as mixing, forming, or rolling,and hence characteristics such as a satisfactory specific dielectricconstant and a satisfactory expansion coefficient can be secured.

In addition, the inventors have revealed the following new problem: whenthe board or a material therefor is exposed to a treatment liquid havinghigh permeability as described in the foregoing, the treatment liquidpermeates the inside of the board or the material to cause an appearancefailure of the board or changes in characteristics thereof. In addition,the inventors have found that, when the critical liquid-repellenttension of the composite material is controlled to 34.0 mN/m or less byadjusting the surface composition, surface structure, or the like of theinorganic fine particle aggregates or the composite material, thecomposite material shows a low specific dielectric constant, and hardlycauses an appearance failure or changes in characteristics even whenexposed to, for example, a treatment liquid to be used in the productionof an electronic circuit board.

The physical properties and characteristics of the“polytetrafluoroethylene”, the “filler”, and the “composite material”,the form and structure of the “composite material”, the applications ofthe “composite material”, a method of producing the “compositematerial”, and the like are described in detail below.

Polytetrafluoroethylene (PTFE)

The composite material is a plate-like material including thepolytetrafluoroethylene and the filler, and the polytetrafluoroethylenein the composite material is preferably “fibrillated”. Fibrils in thefibrillated polytetrafluoroethylene are more preferably oriented notonly in one direction but also in a plurality of directions, and thefibrils and the inorganic fine particle aggregates to be described laterare particularly preferably linked to each other to form a“three-dimensional fine network structure” as shown in images taken witha SEM shown in FIGS. 2A and 2B. When the polytetrafluoroethylene in thecomposite material is fibrillated, in particular, when thethree-dimensional fine network structure is formed, the compositematerial can secure an excellent mechanical strength and excellentdimensional stability. The fibrillation or the like of thepolytetrafluoroethylene may be confirmed through the observation of itssurface with a SEM or the like. In addition, the fibrillation of thepolytetrafluoroethylene, which may be advanced by, for example, applyinga shear force, is more specifically performed through multi-stagerolling to be described later. In addition, the formation of thethree-dimensional fine network structure is performed through, forexample, directionally different multi-stage rolling to be describedlater.

Filler

The composite material is a plate-like material including thepolytetrafluoroethylene and the filler, and one feature thereof lies inthat the filler contains the porous inorganic fine particle aggregateseach formed by the aggregation of the inorganic fine particles having anaverage primary particle diameter of from 5 nm to 200 nm. Each of theporous inorganic fine particle aggregates is specifically such anaggregate as shown in an image taken with a SEM shown in FIG. 3, andmeans an aggregate that is formed by the fusion of a plurality ofinorganic fine particles, and that has voids between the inorganic fineparticles and is hence porous. The term “porous” of the porous inorganicfine particle aggregates as used herein means the voids between theinorganic fine particles forming the aggregates.

Examples of a material for the inorganic fine particles include: anoxide (including a composite oxide) of a typical element, such assilicon oxide (e.g., silicon monoxide or silicon dioxide (silica)) oraluminum oxide (alumina); an oxide (including a composite oxide) of atransition metal, such as titanium oxide (e.g., titanium dioxide(titania)), iron oxide, or zirconium oxide (zirconium dioxide(zirconia)); and a nitride of a typical element, such as boron nitrideor silicon nitride. Those materials may be used alone or in combinationthereof. Of those, an oxide of a typical element is preferred, andsilicon dioxide (silica) is particularly preferred. The oxide of atypical element can suppress the specific dielectric constant of thecomposite material to an extremely low level, and enables the productionof the composite material at lower cost. Although the crystallinity ofthe inorganic fine particles is not particularly limited, when silicondioxide is used, the inorganic fine particles are typically amorphous.

The average primary particle diameter of the inorganic fine particles,which is from 5 nm to 200 nm, is preferably 10 nm or more, morepreferably 15 nm or more, still more preferably 20 nm or more, and ispreferably 150 nm or less, more preferably 120 nm or less, still morepreferably 100 nm or less, particularly preferably 80 nm or less, mostpreferably 70 nm or less. In the case where the average primary particlediameter falls within the range, the inorganic fine particle aggregatesare hardly broken even when subjected to treatment such as mixing,forming, or rolling, and hence satisfactory voids can be secured betweenthe inorganic fine particles. In addition, the plate-like compositematerial easily secures a smooth surface. The average primary particlediameter of the inorganic fine particles is a numerical value obtainedby: measuring the diameters of the particles through their observationwith a SEM; and averaging the measured values. A specific proceduretherefor is as follows: the inorganic fine particles (100 fineparticles) are randomly selected, and their respective particlediameters (the lengths of the long sides of the particles) are measured,followed by the averaging of the resultant particle diameters to providethe numerical value.

The average particle diameter of the primary aggregate products of theinorganic fine particles is typically 100 nm or more, preferably 120 nmor more, more preferably 150 nm or more, and is typically 400 nm orless, preferably 380 nm or less, more preferably 350 nm or less.

The average particle diameter of the secondary aggregate products(aggregate products of the primary aggregate products) of the inorganicfine particles is typically 0.1 μm or more, preferably 1 μm or more,more preferably 2 μm or more, and is typically 100 μm or less,preferably 90 μm or less, more preferably 80 μm or less.

The inorganic fine particle aggregates in the composite material areeach preferably in a state of a secondary aggregate product. When theaggregates are each in a state of a secondary aggregate product, thethree-dimensional fine network structure is easily formed.

In addition, the average particle diameter of the primary aggregateproducts of the inorganic fine particles and the average particlediameter of the secondary aggregate products of the inorganic fineparticles may each be calculated by the same method as that for theaverage primary particle diameter of the inorganic fine particlesdescribed in the foregoing.

The BET specific surface area of the inorganic fine particle aggregatesis typically 10 m²/g or more, preferably 20 m²/g or more, morepreferably 30 m²/g or more, still more preferably 40 m²/g or more, andis typically 250 m²/g or less, preferably 240 m²/g or less, morepreferably 210 m²/g or less, still more preferably 150 m²/g or less,particularly preferably 80 m²/g or less. When the BET specific surfacearea falls within the range, the composite material can secure a highporosity, and an increase in loss tangent thereof can be suppressed. Inparticular, when the BET specific surface area is excessively high, theloss tangent of the composite material tends to increase. The BETspecific surface area of the inorganic fine particle aggregates is anumerical value calculated by substituting, for example, a gasadsorption amount measured by a gas adsorption method (in particular, anitrogen adsorption isotherm) into a BET equation, and is represented bya numerical value before the use of the aggregates in the production ofthe composite material.

The apparent specific gravity of the inorganic fine particle aggregatesis typically 10 g/L or more, preferably 20 g/L or more, more preferably30 g/L or more, still more preferably 40 g/L or more, and is typically100 g/L or less, preferably 90 g/L or less, more preferably 80 g/L orless, still more preferably 70 g/L or less, particularly preferably 60g/L or less. When the apparent specific gravity falls within the range,the composite material can secure a high porosity, and the inorganicfine particle aggregates are hardly broken. The apparent specificgravity of the inorganic fine particle aggregates is a numerical valueobtained by: loading the inorganic fine particle aggregates into acontainer that can measure a volume, such as a 250-milliliter measuringcylinder; measuring the loaded mass (X g) and loaded volume (Y mL) ofthe inorganic fine particle aggregates; and dividing the loaded mass bythe loaded volume ([apparent specific gravity (g/L)]=X/Y×1000″).

Examples of the inorganic fine particle aggregates include MIZUKASILseries (manufactured by Mizusawa Industrial Chemicals, Ltd.), SILYSIAseries (manufactured by Fuji Silysia Chemical Ltd.), hydrophobic AEROSILseries (manufactured by Nippon Aerosil Co., Ltd.), and Nipsil series(manufactured by Tosoh Silica Corporation). Of those, hydrophobic fumedsilica of hydrophobic AEROSIL series (manufactured by Nippon AerosilCo., Ltd.) is particularly preferred.

Although the filler may contain a substance except the inorganic fineparticle aggregates, the content of the inorganic fine particleaggregates in the entirety of the filler is typically 60 mass % or more,preferably 70 mass % or more, more preferably 80 mass % or more, stillmore preferably 90 mass % or more, particularly preferably 100 mass %.When the content falls within the range, the composite material cansecure a high porosity, and the inorganic fine particle aggregates arehardly broken.

Examples of the filler except the inorganic fine particle aggregatesinclude a granular filler and a fibrous filler. Examples of the granularfiller include: solid carbon, such as carbon black; silicon dioxide(silica), such as molten silica or silica gel; an oxide (including acomposite oxide) of a transition metal, such as titanium oxide (e.g.,titanium dioxide (titania)), iron oxide, or zirconium oxide (zirconiumdioxide (zirconia)); and a nitride of a typical element, such as boronnitride or silicon nitride. Examples of the fibrous filler include glassfiber and carbon fiber. Those fillers may be used alone or incombination thereof.

The hydrophobic degree of the filler (containing the inorganic fineparticle aggregates) may be identified by the test of powder for itswettability with an aqueous solution of methanol. The test of the powderfor its wettability is an approach involving spreading the powder in theaqueous solution of methanol at 25° C. to determine the concentration ofmethanol in the aqueous solution of methanol when the floating amount ofthe powder becomes 0 mass %. The filler hydrophobized to a larger extenttends to be less likely to precipitate in water, and to be more likelyto precipitate therein as the methanol concentration increases.Therefore, a lower methanol concentration when the floating amount ofthe powder becomes 0 mass % means that the filler is hydrophobized to alarger extent.

The hydrophobic degree (the methanol concentration in the test of thepowder for its wettability) of the filler (containing the inorganic fineparticle aggregates) is typically 70 mass % or less, preferably 65 mass% or less, more preferably 60 mass % or less, still more preferably 55mass % or less, particularly preferably 50 mass % or less, and istypically 30 mass % or more.

The content of the filler in the composite material is typically 30parts by mass or more, preferably 40 parts by mass or more, morepreferably 45 parts by mass or more, still more preferably 50 parts bymass or more, particularly preferably 55 parts by mass or more withrespect to 100 parts by mass of the total of the polytetrafluoroethyleneand the filler, and is typically 85 parts by mass or less, preferably 80parts by mass or less, more preferably 75 parts by mass or less, stillmore preferably 70 parts by mass or less, particularly preferably 65parts by mass or less with respect thereto. When the content fallswithin the range, the composite material can secure characteristics suchas a satisfactory specific dielectric constant and a satisfactoryexpansion coefficient. The treatment liquid easily permeates thecomposite material having a high content of the filler, in particular, ahigh content of the inorganic fine particle aggregates, and hence thepresent disclosure can be more effectively utilized.

Although the composite material may contain a substance except thepolytetrafluoroethylene and filler described in the foregoing, the totalcontent of the polytetrafluoroethylene and the filler in the entirety ofthe composite material is typically 60 mass % or more, preferably 70mass % or more, more preferably 80 mass % or more, still more preferably90 mass % or more, particularly preferably 100 mass %.

Physical Properties and Characteristics of Composite Material

One feature of the composite material lies in that its porosity is 35%or more. The porosity of the composite material is preferably 40% ormore, more preferably 45% or more, still more preferably 50% or more,particularly preferably 55% or more, and is typically 80% or less,preferably 70% or less. When the porosity falls within the range, thecomposite material can secure characteristics such as a satisfactoryspecific dielectric constant and a satisfactory expansion coefficient.In particular, the treatment liquid easily permeates the compositematerial having a high porosity, and hence the present disclosure can bemore effectively utilized. The porosity of the composite material is anumerical value calculated by: measuring the volume of the compositematerial, the specific gravity and mass (blending mass) of thepolytetrafluoroethylene (PTFE), and the specific gravity and mass(blending mass) of the inorganic fine particle aggregates; andsubstituting the measured values into the following equation.

[Porosity(%)]=([volume of composite material]−[mass of PTFE/specificgravity of PTFE]−[mass of inorganic fine particle aggregates/specificgravity of inorganic fine particle aggregates])/[volume of compositematerial]×100

One feature of the composite material lies in that its criticalliquid-repellent tension is 34.0 mN/m or less. The criticalliquid-repellent tension of the composite material is preferably 33.0mN/m or less, more preferably 32.0 mN/m or less, still more preferably31.0 mN/m or less, particularly preferably 30.0 mN/m or less, and thelower limit value thereof is typically 22.6 mN/m. When the criticalliquid-repellent tension falls within the range, the occurrence of anappearance failure or changes in characteristics due to, for example,the treatment liquid to be used in the production of an electroniccircuit board is particularly suppressed. The following wetting tensiontest is described in detail below.

[Wetting Tension Test]

A test object is immersed in each of test mixtures, which correspond totest mixtures described in JIS K 6768:1999 of Japan IndustrialStandards, and which have wetting tensions at 23° C. of 22.6 mN/m, 25.4mN/m, 27.3 mN/m, 30.0 mN/m, 31.0 mN/m, 32.0 mN/m, 33.0 mN/m, 34.0 mN/m,35.0 mN/m, 36.0 mN/m, 37.0 mN/m, 38.0 mN/m, 39.0 mN/m, 40.0 mN/m, 41.0mN/m, 42.0 mN/m, 43.0 mN/m, 44.0 mN/m, 45.0 mN/m, 46.0 mN/m, 48.0 mN/m,and 50.0 mN/m, at 25° C. for 1 minute to confirm whether or not each ofthe test mixtures permeates the test object, and a numerical value of awetting tension of a test mixture having the smallest wetting tensionout of the test mixtures that have not permeated the test object isdetermined to be a critical liquid-repellent tension of the test object.

The test mixtures corresponding to the test mixtures described in JIS K6768:1999 of Japan Industrial Standards are, for example, “WETTINGTENSION TEST MIXTURES” manufactured by Wako Pure Chemical Industries,Ltd. JIS K 6768:1999 is a standard after revision based on ISO 8296 ofthe International Organization for Standardization. The WETTING TENSIONTEST MIXTURES come in 36 kinds having different wetting tensions (at 23°C.) (in the range of from 22.6 mN/m to 73.0 mN/m), and each contain acolorant, and hence whether or not the mixtures permeate the test objectmay be visually confirmed with ease. In addition, the fact that any oneof the test mixtures does not permeate the test object may be judgedfrom the fact that a mass change ratio after the immersion is less than1% (priority is placed on the judgment based on the mass change ratio).A method of determining the critical liquid-repellent tension isdescribed by way of a specific example. When a wetting tension testmixture having a wetting tension of 33.0 mN/m or less permeates the testobject, and a wetting tension test mixture having a wetting tension of34.0 mN/m or more does not permeate the test object, the numerical valueof the wetting tension of the test mixture having the smallest wettingtension that has not permeated the test object is 34.0 mN/m, and hencethe critical liquid-repellent tension of the test object is 34.0 mN/m.The immersion of the test object in a wetting tension test mixture thatobviously fails to permeate the test object or a wetting tension testmixture that obviously permeates the test object may be appropriatelyomitted.

A method of controlling the critical liquid-repellent tension of thecomposite material to 34.0 mN/m or less is not particularly limited, anda known technology may be appropriately adopted; specific examplesthereof include the following method (i) and method (ii):

(i) a method involving modifying the surface of the filler (containingthe inorganic fine particle aggregates) with a surface modifier having ahydrophobic group (hereinafter sometimes abbreviated as “surfacemodifier”); and(ii) a method involving forming a fine structure on the surface of thefiller.

The “method (i)”, the “method (ii)”, and the like are described indetail below.

Examples of the hydrophobic group of the surface modifier in the method(i) include a fluoro group (—F) and a hydrocarbon group (—C_(n)H_(2n+1)(n=1 to 30)). Of those, a fluoro group that exhibits a liquid-repellingproperty not only on water but also on an oil agent is particularlypreferred.

The surface modifier may be a surface modifier that chemically adsorbs(reacts) to the surface of the filler, or may be a surface modifier thatphysically adsorbs to the surface of the filler, and may be alow-molecular weight compound, or may be a polymer compound. The surfacemodifier that chemically adsorbs (reacts) to the surface of the fillertypically has a reactive functional group that reacts with a surfacefunctional group (e.g., a hydroxyl group (—OH)) of the filler, andexamples of the reactive functional group include an alkoxysilyl group(—SiOR, where the number of carbon atoms of R is from 1 to 6), achlorosilyl group (—SiCl), a bromosilyl group (—SiBr), and a hydrosilylgroup (—SiH). A known method may be appropriately adopted as a method ofmodifying the surface of the filler with the surface modifier, and is,for example, a method involving bringing the filler and the surfacemodifier into contact with each other.

The number of kinds of the surface modifiers is not limited to one, andtwo or more kinds of the surface modifiers may be used in combination.For example, after a surface modifier serving as a low-molecular weightcompound having a reactive functional group has been caused to reactwith the surface of the filler, a surface modifier that is a polymercompound having a hydrophobic group may be caused to physically adsorbonto the resultant. In the case where a material for the filler issilicon dioxide (silica) or the like, the filler may be dissolved(decomposed) when exposed to a basic aqueous solution. However, when thefiller is modified as described above, its resistance to the basicaqueous solution can be improved.

The thermal decomposition temperature of the surface modifier istypically 250° C. or more, preferably 300° C. or more, more preferably350° C. or more, still more preferably 360° C. or more, particularlypreferably 370° C. or more. In the case where the thermal decompositiontemperature falls within the range, the surface modifier can besuppressed from decomposing even when subjected to treatment such ashigh-temperature heating. The thermal decomposition temperature of thesurface modifier is defined as the temperature at which the weight ofthe surface modifier reduces by 5% when its temperature is increased bya thermal weight reduction analysis method (TG-DTA) at 20° C./min.

Examples of a surface modifier that is a low-molecular weight compoundhaving a fluoro group and a reactive functional group include compoundsrepresented by the following formulae. The compounds represented by thefollowing formulae are commercially available, and may each beappropriately obtained and utilized as a surface modifier.

Examples of a surface modifier that is a polymer compound having afluoro group include compounds represented by the following formulae. Inthe following formulae, n and m each represent a positive number of 1 ormore.

A solution commercially available as a surface modifier may be utilized,and a suitable example thereof is NOVEC (trademark) 2202 manufactured by3M Company. It has been announced that the NOVEC (trademark) 2202contains a polymer compound having a fluoro group and is blended with a“fluoroalkylsilane polymer”. The use of the NOVEC (trademark) 2202 as asurface modifier has an advantage in that the critical liquid-repellenttension of the composite material is easily suppressed to a low level bya relatively simple operation.

The content of the surface modifier (organic matter content) in thefiller is typically 0.1 mass % or more, preferably 1 mass % or more,more preferably 2 mass % or more, still more preferably 3 mass % ormore, particularly preferably 4 mass % or more, and is typically 50 mass% or less, preferably 40 mass % or less, more preferably 30 mass % orless, still more preferably 25 mass % or less, particularly preferably20 mass % or less. When the content falls within the range, the criticalliquid-repellent tension of the composite material is easily suppressedto a low level, and hence the linear thermal expansion coefficient andloss tangent thereof are easily suppressed to low levels.

Examples of the fine structure in the method (ii) include a “glasssurface fine structure” described in JP-A-2008-239429 and a“confetti-like projection” described in JP-A-2012-219004. When the finestructure is formed on the surface of the filler, anultra-water-repelling and oil-repelling property is expressed, and hencethe critical liquid-repellent tension of the composite material can becontrolled to 34.0 mN/m or less. Examples of a method of forming thefine structure include methods described in those literatures.

The other physical properties, characteristics, and the like of thecomposite material are not particularly limited as long as its porosityis 35% or more, and its critical liquid-repellent tension is 34.0 mN/mor less. The specific dielectric constant, loss tangent, linear thermalexpansion coefficient, and the like of the composite material aredescribed below byway of their preferred numerical ranges.

The specific dielectric constant (frequency: 10 GHz) of the compositematerial is typically 2.0 or less, preferably 1.90 or less, morepreferably 1.8 or less, still more preferably 1.75 or less, particularlypreferably 1.70 or less, and is typically 1.55 or more. The specificdielectric constant of the composite material is the numerical value ofa real part (εr′) calculated by measuring a complex dielectric constantin accordance with a cavity resonator perturbation method (measurementfrequency: 10 GHz).

The loss tangent (frequency: 10 GHz) of the composite material istypically 0.01 or less, preferably 0.008 or less, more preferably 0.006or less, still more preferably 0.004 or less, particularly preferably0.002 or less, and is typically 0.0005 or more. The loss tangent of thecomposite material is the ratio (εr″/εr′) of an imaginary part (εr″) tothe real part (εr′) calculated by measuring the complex dielectricconstant in accordance with the cavity resonator perturbation method(measurement frequency: 10 GHz).

The linear thermal expansion coefficient of the composite material istypically 70 ppm/K or less, preferably 60 ppm/K or less, more preferably55 ppm/K or less, still more preferably 50 ppm/K or less, particularlypreferably 45 ppm/K or less, and is typically 10 ppm/K or more. Thelinear thermal expansion efficient of the composite material is thenumerical value of an average linear thermal expansion coefficient inthe range of from −50° C. to 200° C. obtained by a thermal mechanicalanalysis (TMA) method. Specifically, the composite material having awidth of 4 mm and a length of 20 mm is fixed in its lengthwisedirection, and a load of 2 g is applied thereto. The temperature of thematerial is increased from room temperature (23° C.) to 200° C. at arate of temperature increase of 10° C./min, and is held at 200° C. for30 minutes so that the residual stress of the material may be removed.Next, the temperature is cooled to −50° C. at 10° C./min, and is held at−50° C. for 15 minutes. After that, the temperature is increased to 200°C. at 2° C./min. An average linear thermal expansion coefficient in therange of from −50° C. to 200° C. in the second temperature increaseprocess was defined as the linear thermal expansion coefficient.

Form and Structure of Composite Material

The composite material is of a plate shape, and the thickness of thecomposite material is typically 0.01 mm or more, preferably 0.02 mm ormore, more preferably 0.05 mm or more, still more preferably 0.07 mm ormore, particularly preferably 0.08 mm or more, and is typically 2.0 mmor less, preferably 1.0 mm or less, more preferably 0.5 mm or less,still more preferably 0.2 mm or less, particularly preferably 0.15 mm orless.

The composite material may include any other layer on one surface, oreach of both surfaces, thereof. Examples of the other layer include ametal layer and a resin layer. The metal layer is utilized in a wiringor the like when the composite material is used as, for example, anelectronic circuit board. The resin layer may be used for variouspurposes; for example, when the composite material, the resin layer, andthe metal layer are laminated in the stated order, the resin layer isutilized as an adhesion layer.

In general, examples of metallic species of the metal layer when thelayer is used as a wiring include gold (Au), silver (Ag), platinum (Pt),copper (Cu), aluminum (Al), and alloys containing these metallicspecies.

The thickness of the metal layer when the layer is used as a wiring istypically 5 μm or more, preferably 10 μm or more, more preferably 15 μmor more, and is typically 50 μm or less, preferably 45 μm or less, morepreferably 40 μm or less.

In general, the kind of the resin layer when the layer is used as anadhesion layer is a thermoplastic resin, preferably a fluorine-basedresin, more preferably a perfluoroalkoxyalkane (PFA, melting point: 310°C.), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP, meltingpoint: 260° C.), polychlorotrifluoroethylene (PCTEF, melting point: 220°C.), a tetrafluoroethylene-ethylene copolymer (ETFE, melting point: 270°C.), a chlorotrifluoroethylene-ethylene copolymer (ECTFE, melting point:270° C.), and polyvinylidene fluoride (PVDF, melting point: 151° C. to178° C.), each of which has a melting point lower than that ofpolytetrafluoroethylene (PTFE, melting point: 327° C.), particularlypreferably PFA.

Applications of Composite Material

Although the applications of the composite material are not particularlylimited, preferred examples thereof include electronic circuit boards,more preferred examples thereof include the wiring boards of the modulesof a cellular phone, a computer, an antenna, and the like, andparticularly preferred examples thereof include the wiring boards ofmillimeter-wave antennas (wiring boards for high frequencies). In theapplications of the wiring boards of the millimeter-wave antennas, thecharacteristics of the composite material, such as a specific dielectricconstant, can be effectively utilized.

Method of Producing Composite Material

The composite material is a plate-like material including thepolytetrafluoroethylene and the filler, but a method of producing thecomposite material is not particularly limited, and the material may beproduced by appropriately adopting a known finding. A method ofproducing the composite material including the followingfiller-preparing step, mixing step, forming step, and rolling step(hereinafter sometimes abbreviated as “method of producing the compositematerial”) is particularly preferred:

-   -   a filler-preparing step of preparing a filler containing porous        inorganic fine particle aggregates each formed by the        aggregation of inorganic fine particles having an average        primary particle diameter of from 5 nm to 200 nm (hereinafter        sometimes abbreviated as “filler-preparing step”);    -   a mixing step of mixing polytetrafluoroethylene and the filler        to provide a precursor composition (hereinafter sometimes        abbreviated as “mixing step”);    -   a forming step of forming the precursor composition to provide a        product to be rolled that can be rolled (hereinafter sometimes        abbreviated as “forming step”); and    -   a rolling step of rolling the product to be rolled to provide a        rolled product (hereinafter sometimes abbreviated as “rolling        step”).

The “filler-preparing step”, the “mixing step”, the “forming step”, the“rolling step”, and the like are described in detail below.

The filler-preparing step is a step of preparing the filler containingthe inorganic fine particle aggregates, but the filler (containing theinorganic fine particle aggregates) may be obtained from the market, ormay be produced by oneself; provided that the filler-preparing steppreferably includes performing at least one selected from the groupconsisting of the method (i) and the method (ii) on the filler in orderthat the critical liquid-repellent tension of the composite material maybe controlled to 34.0 mN/m or less.

The mixing step is a step of mixing the polytetrafluoroethylene and thefiller to provide the precursor composition, and thepolytetrafluoroethylene is preferably a fine powder product or agranulated product.

The average particle diameter of the fine powder product or granulatedproduct of the polytetrafluoroethylene is preferably larger than theaverage primary particle diameter of the inorganic fine particleaggregates, that is, from 5 nm to 200 nm, and is particularly preferablymore than 0.2 μm and 650 μm or less. The average particle diameter ofthe fine powder product or the granulated product may be determined inconformity with JIS K 6891-5.4.

In the mixing step, in addition to the polytetrafluoroethylene and thefiller, a solvent and a volatile additive are preferably added beforethe mixing. The solvent has an action of bringing the precursorcomposition into a paste state to enable its uniform dispersion, and thevolatile additive has the following action: the additive is finallyremoved by volatilization to increase the porosity of the compositematerial.

Examples of the solvent include water and lower alcohols, such asmethanol, ethanol, isopropanol, and butanol.

The volatile additive means a compound that has a boiling point of from30° C. to 300° C. and is a liquid at room temperature, and the boilingpoint of the volatile additive is preferably 50° C. or more, morepreferably 100° C. or more, still more preferably 200° C. or more, andis preferably 280° C. or less, more preferably 260° C. or less, stillmore preferably 240° C. or less.

Examples of the kind of the volatile additive include a hydrocarbon, anether, an ester, and an alcohol each having low reactivity, and analiphatic saturated hydrocarbon is preferred. Specific examples thereofinclude hexane (boiling point: 69° C.), heptane (boiling point: 98° C.),octane (boiling point: 126° C.), nonane (boiling point: 151° C.), decane(boiling point: 174° C.), undecane (boiling point: 196° C.), dodecane(boiling point: 215° C.), tridecane (boiling point: 234° C.), andtetradecane (boiling point: 254° C.), and dodecane is particularlypreferred.

The addition amount of the volatile additive is typically 1 part by massor more, preferably 5 parts by mass or more, more preferably 10 parts bymass or more, still more preferably 20 parts by mass or more,particularly preferably 30 parts by mass or more with respect to 100parts by mass of the total of the polytetrafluoroethylene and thefiller, and is typically 200 parts by mass or less, preferably 150 partsby mass or less, more preferably 130 parts by mass or less, still morepreferably 110 parts by mass or less, particularly preferably 100 partsby mass or less with respect thereto. When the addition amount fallswithin the range, the composite material can secure a satisfactoryporosity.

The forming step is a step of forming the precursor composition toprovide the product to be rolled that can be rolled, and a formingmachine to be used in the forming step is, for example, an FT die, aPress machine, an extrusion molding machine, or a calender roll. Ofthose, an FT die is particularly preferred.

The rolling step is a step of rolling the product to be rolled toprovide the rolled product. The step is preferably “multi-stage rolling”in which the following operation is repeated a plurality of times: theresultant rolled products are laminated, and the laminate is rolled as aproduct to be rolled. The step is particularly preferably “directionallydifferent multi-stage rolling” in which the product to be rolled isrolled in a direction different from the previous rolling direction. Thedirectionally different multi-stage rolling is, for example, therepetition of the following operation: a product to be rolled isobtained by laminating rolled products so that the rolled products mayface the same rolling direction, and the product to be rolled is rolledin a rolling direction rotated from the previous rolling direction by90°.

The number of the rolled products to be laminated in the multi-stagerolling is typically 2 or more, preferably 3 or more, more preferably 4or more, still more preferably 10 or more, particularly preferably 30 ormore, and is typically 2,000 or less, preferably 1,000 or less, morepreferably 500 or less, still more preferably 200 or less, particularlypreferably 100 or less.

The rolling ratio in the rolling step is typically 10 or more,preferably 20 or more, more preferably 40 or more, still more preferably50 or more, particularly preferably 100 or more, and is typically 20,000or less, preferably 10,000 or less, more preferably 5,000 or less, stillmore preferably 2,000 or less, particularly preferably 1,000 or less.

A rolling ratio λ is determined from the equation “λ=L2/L1” by using thelength (L1) of a raw sheet and the length (L2) thereof after itsrolling.

An apparatus to be used in the rolling step is, for example, a pressmachine, an extrusion molding machine, or a rolling roll (e.g., acalender roll).

The method of producing the composite material may include any otherstep, and specific examples thereof include the following steps:

-   -   an additive-removing step of removing the volatile additive from        the rolled product (hereinafter sometimes abbreviated as        “additive-removing step”);    -   a heating and pressurizing step of heating and pressurizing the        rolled product (hereinafter sometimes abbreviated as “heating        and pressurizing step”);    -   an other layer-forming step of forming at least one selected        from the group consisting of a metal layer and a resin layer on        one surface, or each of both surfaces, of the rolled product        (hereinafter sometimes abbreviated as “other layer-forming        step”); and    -   a patterning step of subjecting the metal layer to patterning        treatment (hereinafter sometimes abbreviated as “patterning        step”).

The “additive-removing step”, the “heating and pressurizing step”, the“other layer-forming step”, the “patterning step”, and the like aredescribed in detail below.

The additive-removing step is a step of removing the volatile additivefrom the rolled product, and a method therefor is typically, forexample, a method involving heating the rolled product in a heatingfurnace that may be used in drying. A heating condition may beappropriately selected in accordance with, for example, the boilingpoint of the volatile additive.

The heating and pressurizing step is a step of heating and pressurizingthe rolled product, and a method therefor is typically, for example, amethod involving heating and pressurizing the rolled product with apress machine or the like. A heating temperature is typically from 300°C. to 500° C., and pressurization conditions are typically as follows:the rolled product is pressurized at from 0.2 MPa to 30 MPa for from 5minutes to 60 minutes.

The other layer-forming step is a step of forming at least one selectedfrom the group consisting of the resin layer and the metal layer on onesurface, or each of both surfaces, of the rolled product, and a methodof forming the metal layer is, for example, sputtering, plating, thepressure bonding of a metal foil, or a lamination method. A method offorming the resin layer is, for example, a method involving sandwichinga resin film between the composite material and a metal foil, andsubjecting the resultant to pressure bonding.

The patterning step is a step of subjecting the metal layer to thepatterning treatment, and a method for the patterning treatment is, forexample, an additive method involving using a photoresist or the like,or a subtractive method based on etching.

EXAMPLES

Next, the composite material of the present disclosure is specificallydescribed by way of Examples, but the present disclosure is not limitedto these Examples.

Example 1

Hydrophilic fumed silica (manufactured by Nippon Aerosil Co., Ltd.,product number: “AEROSIL 50”, BET specific surface area: 50±15 m²/g,apparent specific gravity: 50 g/L, average primary particle diameter: 40nm, average particle diameter of secondary aggregate products: 0.2 μm)was used as inorganic fine particle aggregates, and triethoxy-1H, 1H,2H, 2H-tridecafluoro-n-octylsilane (manufactured by Tokyo ChemicalIndustry Co., Ltd., product number: “T1770”) represented by thefollowing formula was used as a surface modifier. The inorganic fineparticle aggregates were modified with the surface modifier.

Specifically, 40.8 g of the surface modifier, 22.1 g of acetic acid,43.2 g of pure water, and 80 g of the inorganic fine particle aggregateswere added to 832.9 g of isopropanol, and the mixture was stirred for 24hours to provide a dispersion of the inorganic fine particle aggregates.Next, the dispersion was heated at 100° C. for 1 hour, and was furtherheated at 200° C. for hours to provide surface-modified inorganic fineparticle aggregates. The hydrophobic degree (test of powder for itswettability) of the inorganic fine particle aggregates was 68 mass %.

Next, polytetrafluoroethylene, the inorganic fine particle aggregates,and a volatile additive were mixed with a V-type mixer. Specifically,the polytetrafluoroethylene (manufactured by Daikin Industries, Ltd.,product number: “POLYFLON PTFE F-104”, average particle diameter: 550μm) was added in consideration of its solid content so that a mass ratiobetween the polytetrafluoroethylene and the resultant inorganic fineparticle aggregates became 40:60, and dodecane was added as the volatileadditive so that its amount became 50 parts by mass with respect to 100parts by mass of the total of the polytetrafluoroethylene and theinorganic fine particle aggregates. The materials were mixed at a numberof revolutions of 10 rpm and a temperature of 24° C. for 5 minutes toprovide a paste.

The paste was passed through a pair of rolling rolls to provide anelliptical base sheet (sheet-like formed body) having a thickness of 3mm, a width of from 10 mm to 50 mm, and a length of 150 mm, and aplurality of the base sheets were produced. Next, two of the base sheetswere laminated while their rolling directions were aligned with eachother, and the laminate was passed through the rolling rolls in theprevious rolling direction to be rolled. Thus, a first rolled laminatedsheet was produced. A plurality of the first rolled laminated sheetswere produced. Next, two of the first rolled laminated sheets werelaminated while their rolling directions were aligned with each other.The rolled laminate sheets were rotated from the previous rollingdirection by 90° while their sheet surfaces were kept parallel to eachother, and the sheets were passed through the rolling rolls to berolled. Thus, a second rolled laminated sheet was produced. A pluralityof the second rolled laminated sheets were produced. Further, two of thesecond rolled laminated sheets were rolled in the same manner as in themethod of producing the second rolled laminated sheet to produce a thirdrolled laminated sheet. The step of laminating and rolling the sheets asdescribed above was repeated a total of five times counting from thelamination and rolling of the base sheets, and then the resultant wasrolled a plurality of times while a gap between the rolling rolls wasnarrowed by 0.5 mm each time. Thus, a rolled laminated sheet having athickness of about 0.18 mm was obtained (number of layers forming thesheet: 32).

The volatile additive was removed by heating the resultant rolledlaminated sheet at 150° C. for 20 minutes. The resultant sheet wassubjected to pressure forming at 380° C. and 1 MPa for 5 minutes toproduce a plate-like composite material having a thickness of about 0.15mm.

The porosity, hydrophobic degree, critical liquid-repellent tension,specific dielectric constant and loss tangent, and linear thermalexpansion coefficient of the resultant composite material were measuredas described below, and a mass change ratio when the material wasimmersed in a cleaner-conditioner solution was measured. The hydrophobicdegree was measured for a filler. The results are shown in Table 1.

Porosity

The porosity was calculated by: measuring the volume of the compositematerial, the specific gravity and mass (blending mass) of thepolytetrafluoroethylene (PTFE), and the specific gravity and mass(blending mass) of the inorganic fine particle aggregates; andsubstituting the measured values into the following equation.

[Porosity(%)]=([volume of composite material]−[mass of PTFE/specificgravity of PTFE]−[mass of inorganic fine particle aggregates/specificgravity of inorganic fine particle aggregates])/[volume of compositematerial]×100

Hydrophobic Degree

The hydrophobic degree of the filler was calculated by: spreading thepowder of the filler in an aqueous solution of methanol at 25° C.; anddetermining the concentration of methanol in the aqueous solution ofmethanol when the floating amount of the powder became 0 mass %.

Critical Liquid-Repellent Tension

The composite material was immersed in “WETTING TENSION TEST MIXTURE”manufactured by Wako Pure Chemical Industries, Ltd. at room temperaturefor 1 minute, and was then washed with distilled water, followed byvisual judgment of whether or not the wetting tension test mixturepermeated the composite material. WETTING TENSION TEST MIXTURES Nos.22.6 to 50.0 were used, and the numerical value of the surface tensionof the test mixture having the lowest surface tension out of the wettingtension test mixtures that had not been observed to permeate thematerial was adopted as the result of the critical liquid-repellenttension of the material.

Specific Dielectric Constant and Loss Tangent

A complex dielectric constant was measured by a cavity resonatorperturbation method at a measurement frequency of 10 GHz, and its realpart (εr′) was adopted as a specific dielectric constant. In addition, aloss tangent was determined from the ratio (εr″/εr′) of an imaginarypart (εr″) to the real part.

With the use of a specific dielectric constant-measuring apparatus(“Network Analyzer N5230C” manufactured by Agilent Technologies, and“Cavity Resonator 10 GHz” manufactured by Kanto Electronic Applicationand Development Inc.), a strip-shaped sample (sample size: 2 mm inwidth×70 mm in length) was cut out of each sheet and subjected to themeasurement.

Linear Thermal Expansion Coefficient

An average linear thermal expansion coefficient in a sheet planedirection in the range of from −50° C. to 200° C. was determined as alinear thermal expansion coefficient (ppm/K) by a TMA method with athermal mechanical analyzer (manufactured by BRUKER AXS, “TMA4000SA”).Specifically, the composite material having a width of 4 mm and a lengthof 20 mm is fixed in its lengthwise direction, and a load of 2 g isapplied thereto. The temperature of the material is increased from roomtemperature to 200° C. at a rate of temperature increase of 10° C./min,and is held at 200° C. for 30 minutes so that the residual stress of thematerial may be removed. Next, the temperature is cooled to −50° C. at10° C./min, and is held at −50° C. for 15 minutes. After that, thetemperature is increased to 200° C. at 2° C./min. An average linearthermal expansion coefficient in the range of from −50° C. to 200° C. inthe second temperature increase process was defined as the linearthermal expansion coefficient.

Mass Change Ratio by Cleaner-Conditioner Solution (CC Solution)Immersion Test

A cleaner-conditioner solution containing a surfactant for electrolessplating pretreatment (manufactured by Rohm and Haas ElectronicMaterials, CIRCUPOSIT (trademark) CONDITIONER-NEUTRALIZER 3320), whichwas a solution having relatively high permeability to be used inelectronic circuit processing, was used to measure the mass change ratioof the resultant composite material by its immersion in the solution. Aspecific procedure is as follows: the mass of the composite material ismeasured in advance; next, the material is immersed in a mixed solutioncontaining the cleaner-conditioner solution and ion-exchanged water (ata volume ratio of 10:90) at 45° C. for 5 minutes; and the material isrinsed with ion-exchanged water, and moisture on its surface is wipedoff, followed by the measurement of its mass to calculate the masschange ratio.

Example 2

A composite material was produced by the same method as that of Example1 except that dodecane was added so that its amount became 45 parts bymass with respect to 100 parts by mass of the total of thepolytetrafluoroethylene and the inorganic fine particle aggregates. Theresults are shown in Table 1.

Example 3

A composite material was produced by the same method as that of Example1 except that dodecane was added so that its amount became 40 parts bymass with respect to 100 parts by mass of the total of thepolytetrafluoroethylene and the inorganic fine particle aggregates. Theresults are shown in Table 1.

Example 4

A composite material was produced by the same method as that of Example3 except that: the surface modifier was changed to NOVEC (trademark)2202 manufactured by 3M Company; and the method for the surfacemodification was changed as described below. The surface modificationwas performed by mixing and dispersing the inorganic fine particleaggregates so that the solid content of the NOVEC (trademark) 2202became 25 parts by mass with respect to 100 parts by mass of theinorganic fine particle aggregates. The results are shown in Table 1.

Example 5

A composite material was produced by the same method as that of Example4 except that the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with an aminosilane ((C₂H₅O)₃SiC₃H₆NH₂)and hexamethyldisilazane (HMDS) (manufactured by Nippon Aerosil Co.,Ltd., product number: “AEROSIL NA50H”, BET specific surface area: 40±10m²/g, apparent specific gravity: 50 g/L). The results are shown in Table1.

Example 6

A composite material was produced by the same method as that of Example2 except that the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with an aminosilane ((C₂H₅O)₃SiC₃H₆NH₂)and hexamethyldisilazane (HMDS) (manufactured by Nippon Aerosil Co.,Ltd., product number: “AEROSIL NA50H”, BET specific surface area: 40±10m²/g, apparent specific gravity: 50 g/L). The results are shown in Table2.

Comparative Example 1

A composite material was produced by the same method as that of Example1 except that: the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with dimethylpolysiloxane (siliconeoil) (manufactured by Nippon Aerosil Co., Ltd., product number: “AEROSILNY50”, BET specific surface area: 30±10 m²/g, apparent specific gravity:50 g/L, average particle diameter of primary particles: 40 nm); and itssurface was not modified with the surface modifier (triethoxy-1H, 1H,2H, 2H-tridecafluoro-n-octylsilane). The results are shown in Table 2.

Comparative Example 2

A composite material was produced by the same method as that of Example1 except that: the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with hexamethyldisilazane (HMDS)(manufactured by Nippon Aerosil Co., Ltd., product number: “AEROSILNAX50”, BET specific surface area: 40±10 m²/g, apparent specificgravity: 40 g/L, average particle diameter of primary particles: 40 nm);and its surface was not modified with the surface modifier(triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane). The results areshown in Table 2.

Comparative Example 3

A composite material was produced by the same method as that of Example1 except that: the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with dimethylpolysiloxane (siliconeoil) (manufactured by Nippon Aerosil Co., Ltd., product number: “AEROSILRY200S”, BET specific surface area: 80±15 m²/g, apparent specificgravity: 50 g/L, average particle diameter of primary particles: 10 nm);and its surface was not modified with the surface modifier(triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane). The results areshown in Table 2.

Comparative Example 4

A composite material was produced by the same method as that of Example2 except that: the inorganic fine particle aggregates were changed tohydrophobic fumed silica treated with hexamethyldisilazane (HMDS)(manufactured by Nippon Aerosil Co., Ltd., product number: “AEROSILRX300”, BET specific surface area: 200±20 m²/g, apparent specificgravity: 40 g/L, average particle diameter of primary particles: 8 nm);and its surface was not modified with the surface modifier(triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane). The results areshown in Table 2.

Comparative Example 5

A composite material was produced by the same method as that of Example3 except that: the inorganic fine particle aggregates were changed tosilica aerogel; and its surface was not modified with the surfacemodifier. The results are shown in Table 3.

Comparative Example 6

A composite material was produced by the same method as that of Example1 except that: the inorganic fine particle aggregates were changed tohollow silica (manufactured by 3M Company, iM16K); its surface was notmodified with the surface modifier; and dodecane was added so that itsamount became 55 parts by mass with respect to 100 parts by mass of thetotal of the polytetrafluoroethylene and the hollow silica. The resultsare shown in Table 3.

TABLE 1 Example 1 Example 2 Filler Kind AEROSIL50 AEROSIL50 Porousinorganic fine Porous inorganic fine particle aggregates particleaggregates BET specific 50 ± 15 50 ± 15 surface area (m²/g) Apparent50      50      specific gravity (g/L) Kind of surface modifier

Formulation PTFE 40      40      (mass ratio) Filler 60      60     Volative 50      45      additive Physical Porosity (%) 55      49     property Hydrophobic 68      68      degree (mass %) Critical 27.3   27.3    liquid-repellent tension (mN/m) Characteristic Specific  1.7    1.75   evaluation dielectric constant Loss tangent  0.0026  0.0032Linear thermal 47      45      expansion coefficient (ppm/K) Mass change<1%    <1%    ratio by CC solution immersion test Example 3 Example 4Example 5 Filler Kind AEROSIL50 AEROSIL50 AEROSIL NA50H Porous inorganicPorous Porous inorganic fine particle inorganic fine particle aggregatesfine particle aggregates aggregates (treated with (C₂H₅O)₃SiC₃H₆NH₂ andHMDS) BET specific 50 ± 15 50 ± 15 40 ± 10 surface area (m²/g) Apparent50      50      50      specific gravity (g/L) Kind of surface modifier

Novec ™ 2202

Formulation PTFE 40      40      40      (mass ratio) Filler 60     60      60      Volative 40      40      40      additive PhysicalPorosity (%) 44      56      50      property Hydrophobic 68     66      69      degree (mass %) Critical 27.3    27.3    25.4   liquid-repellent tension (mN/m) Characteristic Specific  1.81    1.64   1.74   evaluation dielectric constant Loss tangent  0.0031  0.0019 0.0012 Linear thermal 41      18      23      expansion coefficient(ppm/K) Mass change <1%    <1%    <1%    ratio by CC solution immersiontest

TABLE 2 Comparative Comparative Comparative Comparative Example 6Example 1 Example 2 Example 3 Example 4 Filler Kind AEROSIL NA50HAEROSIL NY50 AEROSIL AEROSIL AEROSIL Porous inorganic Porous inorganicNAX50 RY200S RX300 fine particle fine particle Porous inorganic Porousinorganic Porous inorganic aggregates aggregates fine particle fineparticle fine particle (treated with (treated with aggregates aggregatesaggregates (C₂H₅O)₃SiC₃H₆NH₂ silicone oil) (treated with (treated with(treated with and HMDS) HDMS) silicone oil) HDMS) BET specific 40 ± 1030 ± 10 40 ± 10 80 ± 15 200 ± 20 surface area (m²/g) Apparent 50     50      40      50      40     specific gravity (g/L) Kind of surfacemodifier

Formulation PTFE 40      40      40      40      40     (mass ratio)Filler 60      60      60      60      60     Volative 45      50     50      50      45     additive Physical Porosity (%) 52      57     61      57      63     property Hydrophobic 60      52      47      — —degree (mass %) Critical 34.0    38.0    46.0    40.0    46.0  liquid-repellent tension (mN/m) Characteristic Specific  1.83    1.75   1.68    1.77    1.55  evaluation dielectric constant Loss tangent 0.0049  0.0018  0.0011  0.0036  0.007 Linear thermal — 31      25     38      35     expansion coefficient (ppm/K) Mass change <1%    12.3% 27.9%  15.3%  22.3%  ratio by CC solution immersion test

TABLE 3 Comparative Comparative Example 5 Example 6 Filler Kind Silicaaerogel 1M 16K Hollow silica BET specific surface — — area (m²/g)Apparent specific — — gravity (g/L) Kind of surface modifier — — Form-PTFE 40 40 ulation Filler 60 60 (mass Volatile additive 40 55 ratio)Physical Porosity (%) 67 43 property Hydrophobic — — degree (mass %)Critical liquid-repellent >50.0 34.0 tension (mN/m) Charac- Specificdielectric 1.69 1.63 teristic constant evaluation Loss tangent 0.03450.0068 Linear thermal — 65 expansion coefficient (ppm/K) Mass changeratio 3.1% <1% by CC solution immersion test

It was found that Examples were each a composite material that showed alow specific dielectric constant, and that hardly caused an appearancefailure or changes in characteristics when exposed to the CC solution,and hence Examples were each suitable for an electronic circuit board.In contrast, the mass change ratio of each of Comparative Examples 1 to5 by the CC solution immersion test was large, and Comparative Example 6had a high linear thermal expansion coefficient, and hence ComparativeExamples 1 to 6 were not suitable for practical use.

Although specific modes of the present disclosure have been described inExamples above, Examples are for illustrative purposes only and are notto be construed as limitative. It is intended that various modificationsapparent to a person skilled in the art fall within the scope of thepresent disclosure.

The composite material of the present disclosure is suitable for thewiring boards of the modules of a cellular phone, a computer, anantenna, and the like, in particular, the wiring boards ofmillimeter-wave antennas (wiring boards for high frequencies).

1. A plate-like composite material, comprising: polytetrafluoroethylene;and a filler, wherein the polytetrafluoroethylene is fibrillated,wherein the filler contains porous inorganic fine particle aggregateseach formed by aggregation of inorganic fine particles having an averageprimary particle diameter of from 5 nm to 200 nm, wherein the compositematerial has a porosity of 35% or more, wherein the composite materialhas a critical liquid-repellent tension determined by a wetting tensiontest of 34.0 mN/m or less, wherein, in the wetting tension test, a testobject is immersed in each of test mixtures, which correspond to testmixtures described in JIS K 6768:1999 of Japan Industrial Standards, andwhich have wetting tensions at 23° C. of 22.6 mN/m, 25.4 mN/m, 27.3mN/m, 30.0 mN/m, 31.0 mN/m, 32.0 mN/m, 33.0 mN/m, 34.0 mN/m, 35.0 mN/m,36.0 mN/m, 37.0 mN/m, 38.0 mN/m, 39.0 mN/m, 40.0 mN/m, 41.0 mN/m, 42.0mN/m, 43.0 mN/m, 44.0 mN/m, 45.0 mN/m, 46.0 mN/m, 48.0 mN/m, and 50.0mN/m, at 25° C. for 1 minute to confirm whether or not each of the testmixtures permeates the test object, and a numerical value of a wettingtension of a test mixture having the smallest wetting tension out of thetest mixtures that have not permeated the test object is determined tobe a critical liquid-repellent tension of the test object, and wherein asurface of the filler is modified by a surface modifier having ahydrophobic group or by forming a fine structure on the surface of thefiller.
 2. The composite material according to claim 1, wherein thefiller has a BET specific surface area of from 30 m²/g to 240 m²/g. 3.The composite material according to claim 1, wherein a content of thefiller is 55 parts by mass or more with respect to 100 parts by mass ofa total of the polytetrafluoroethylene and the filler.
 4. The compositematerial according to claim 1, wherein the filler has an apparentspecific gravity of 100 g/L or less.
 5. The composite material accordingto claim 1, wherein the composite material is configured to be anelectronic circuit board.
 6. An electronic circuit board comprising theplate-like composite material of claim 1.